Guide · November 2026

How to cut channel letter backs from ACM

Aluminum composite (ACM) is a common material for channel letter backs. It routs cleanly and stays flat. Most of the waste on a back job comes from how the shapes sit on the sheet.

1. Get the back shapes

Each back is the outline of its letter, with no inside details. Keep the counters (the holes in A, B, D, O, P, R) as separate shapes. They are offcuts you can reuse, and smaller letters can sometimes be cut from them.

• Close every path and remove stray points before you export.
• Check the size against the drawing. A back that is off by a scale factor will not match its return.

2. Nest with the right gap

1. Set part spacing to your bit diameter plus a small margin. See router bit size and part spacing.
2. If the ACM has a brushed or directional finish and the back side will show, limit rotation to 0° or 180° so the grain runs the same way on every letter.
3. If you cut with the finished side down, mirror the layout so the letters read correctly when you flip them.

3. Cut and label

Cut small backs first and hold them with tabs or vacuum so they do not shift. Mark each back with its letter before you take it off the table. It saves a lot of matching later. See how to sort and label letters after the CNC.

Common problems

• Back does not fit the return: the back was cut from a different outline than the return, or the file was scaled. Use the same outline for both.
• Grain runs different ways: rotation was set to any angle. Lock it to 0° or 180° and nest again.
• Edges are rough: check bit wear and feed. ACM needs a sharp bit made for plastics or aluminum composite.

More guides: How to nest channel letters · ACM nesting · FAQ